Ethnic differences in the effects of apolipoprotein E ɛ4 and vascular risk factors on accelerated brain aging

Abstract The frequency of the apolipoprotein E ɛ4 allele and vascular risk factors differs among ethnic groups. We aimed to assess the combined effects of apolipoprotein E ɛ4 and vascular risk factors on brain age in Korean and UK cognitively unimpaired populations. We also aimed to determine the differences in the combined effects between the two populations. We enrolled 2314 cognitively unimpaired individuals aged ≥45 years from Korea and 6942 cognitively unimpaired individuals from the UK, who were matched using propensity scores. Brain age was defined using the brain age index. The apolipoprotein E genotype (ɛ4 carriers, ɛ2 carriers and ɛ3/ɛ3 homozygotes) and vascular risk factors (age, hypertension and diabetes) were considered predictors. Apolipoprotein E ɛ4 carriers in the Korean (β = 0.511, P = 0.012) and UK (β = 0.302, P = 0.006) groups had higher brain age index values. The adverse effects of the apolipoprotein E genotype on brain age index values increased with age in the Korean group alone (ɛ2 carriers × age, β = 0.085, P = 0.009; ɛ4 carriers × age, β = 0.100, P < 0.001). The apolipoprotein E genotype, age and ethnicity showed a three-way interaction with the brain age index (ɛ2 carriers × age × ethnicity, β = 0.091, P = 0.022; ɛ4 carriers × age × ethnicity, β = 0.093, P = 0.003). The effects of apolipoprotein E on the brain age index values were more pronounced in individuals with hypertension in the Korean group alone (ɛ4 carriers × hypertension, β = 0.777, P = 0.038). The apolipoprotein E genotype, age and ethnicity showed a three-way interaction with the brain age index (ɛ4 carriers × hypertension × ethnicity, β=1.091, P = 0.014). We highlight the ethnic differences in the combined effects of the apolipoprotein E ɛ4 genotype and vascular risk factors on accelerated brain age. These findings emphasize the need for ethnicity-specific strategies to mitigate apolipoprotein E ɛ4-related brain aging in cognitively unimpaired individuals.


Introduction
8][9] Notably, the combined effects of APOE ɛ4 and hypertension 7 and APOE ɛ4 and diabetes 8 on cognitive decline are observed in cognitively unimpaired (CU) individuals.
APOE ɛ2 allele is a protective factor against developing Alzheimer's disease. 10,11However, due to the low proportion of APOE ɛ2, studies investigating the association between APOE ɛ2 and brain atrophy are relatively sparse.Consequently, these associations remain controversial.
Previous studies on the impacts of APOE ɛ2 on brain atrophy have linked ɛ2 carriers to relatively less brain atrophy in medial temporal regions. 12,13In contrast, several studies have shown the opposite results that ɛ2 carriers are associated with brain atrophy in the hippocampus. 14,15he trajectory of brain atrophy throughout aging can be captured and translated into an individual's brain age using machine learning algorithms. 16Brain age serves as an indicator of the overall brain health as it allows individual-level inferences rather than group-level assessments.Previous studies have linked vascular risk factors, such as hypertension, diabetes and obesity, to accelerated brain aging in CU populations. 17,18Studies conducted in European and American populations have explored the relationship between APOE ɛ4 and brain age and reported the adverse effects of APOE ɛ4 on brain age. 19,20However, there is currently a gap in the literature regarding the occurrence of these effects in Asian populations and whether the impact of APOE ɛ4 differs among various racial and ethnic groups.
][27] Conversely, vascular risk factors and their associated complications are more prevalent in Asian populations than in European populations. 28,29Our recent study indicated that ethnicity modulates the effects of vascular risk factors on brain age, 30 with a greater impact observed in the Korean population than in the UK population. 30Given the differences in the frequency of APOE ɛ4 carriers, vascular risk factors and their influence on brain health among ethnic groups, we hypothesized that there may be disparities in the combined effects of APOE ɛ4 and vascular risk factors on brain age across ethnic groups.
In this study, we aimed to investigate the relationships between APOE ɛ4, APOE ɛ2, vascular risk factors and brain age measured using the brain age index (BAI) in Korean and UK CU individuals.First, we examined whether APOE ɛ4 or ɛ2 increased the BAI values in both populations.Second, we assessed the combined effects of APOE ɛ4 or ɛ2 and vascular risk factors on the BAI values in both populations.Finally, we determined the potential differences in these combined effects between the two populations.

Study populations
Individuals aged ≥45 years were recruited from the Health Promotion Center at the Samsung Medical Center (SMC-HPC, Seoul, Korea) and underwent a comprehensive battery of health screening examinations between 1 September 2008 and 31 October 2019.A total of 4782 eligible candidates identified as CU underwent a full medical examination comprising cognitive assessment, APOE genotyping and 3.0-T MRI examination, including a high-resolution T 1 -weighted MRI examination, as part of the standard screening for dementia.Participants were excluded if they met the following criteria: 728 had missing data on years of education or Mini-Mental State Examination score 31 ; 509 showed significant cognitive impairment determined either by a Mini-Mental State Examination score below the 16th percentile in age-, sex-and education-matched norms or through an interview with a qualified neurologist; 312 exhibited severe cerebral white matter hyperintensities (deep white matter ≥ 2.5 cm and caps or band ≥ 1.0 cm) or structural brain lesions, such as territorial infarction, lobar hemorrhage, brain tumor and hydrocephalus; 542 had missing data on DM, hypertension or body mass index; and 377 had unreliable cortical thickness analysis due to head motion, blurred MRI, improper registration to a standardized stereotaxic space, misclassification of tissue type or imprecise surface extraction.Finally, 2314 participants were included in this study.
Similarly, data from individuals of British ancestry were sourced from the UK Biobank (UKB, http://www.ukbiobank.ac.uk).Only white adults, such as British (93.7%),Irish (2.77%) and other backgrounds (3.44%), were included in the study.Additionally, we excluded subjects with a self-reported or hospital record-based history of dementia, Parkinson's disease or other central nervous system disorders were excluded.Finally, 17 340 CU individuals registered in the UKB were included after applying the inclusion and exclusion criteria, alongside the random selection of participants for brain imaging data processing.
Propensity score matching was performed using a multivariate logistic regression analysis based on age and sex.A total of 2314 Korean individuals were paired with 6942 UK individuals through propensity score matching using the 1:3 nearest-neighbor algorithm with a caliper of 0.25.
This study received approval from the Institutional Review Board of the Samsung Medical Center and followed the ethical guidelines outlined in the Declaration of Helsinki.All participants recruited from the SMC-HPC provided written informed consent.Anonymous and de-identified data from the UKB were utilized for analysis, thereby exempting the present study from obtaining informed consent.

APOE genotyping
For participants from the SMC, genomic DNA was extracted from peripheral blood leukocytes using the Wizard Genomic DNA Purification Kit, following the manufacturer's protocol (Promega, Madison, WI, USA).Genotyping of the single nucleotide polymorphisms within the APOE gene (rs429358 in codon 112 and rs7412 in codon 158) was conducted using TaqMan Single Nucleotide Polymorphism Genotyping Assay (Applied Biosystems, Foster City, CA, USA) on a 7500 Fast Real-Time PCR System (Applied Biosystems), following the manufacturer's instructions.For participants from the UKB, genotype calling was carried out by Affymetrix (now part of Thermo Fisher Scientific) using two closely related purpose-designed arrays.Among the available genome-wide genetic data of 488 377 participants, 49 950 participants were analysed using the UK BiLEVE Axiom array (Resource 149600), and the remaining 438 427 were assessed using the UKB Axiom array (Resource 149601), demonstrating a 95% marker content similarity between two arrays (ref: 10.1038/s41586-018-0579-z).
A total of 12 participants from the Samsung Medical Center and 413 participants registered in the UKB had the APOE ɛ2/ɛ4 genotype, who were excluded from the main analysis because of the putative opposing effects of ɛ2 and ɛ4 alleles. 25The APOE genotype was classified into three categories: ɛ2 carriers (ɛ2/ɛ2 and ɛ2/ɛ3), ɛ3 homozygotes (ɛ3/ɛ3) and ɛ4 carriers (ɛ3/ɛ4 and ɛ4/ɛ4).

Measurement of vascular risk factors
Age, hypertension and diabetes were considered vascular risk factors.For individuals from the Samsung Medical Center, a health screening examination was conducted by a welltrained medical professional, following standardized protocols, as explained in a previous study. 30Assessment of hypertension and/or diabetes was based on the medical history of hypertension and/or diabetes or current medication usage, such as any antihypertensive and/or antidiabetic medication.For individuals registered in the UKB, their hypertension and diabetes states were determined through a combination of a touchscreen-based questionnaire, verbal interviews and linked hospital records, as outlined in our previous study. 30

Acquisition of brain MRI and image processing
All individuals from the Samsung Medical Center underwent a 3D volumetric brain MRI examination.3D T 1 turbo field echo MRI data were acquired using an Achieva 3.0-T MRI scanner (Philips, Best, the Netherlands) with the following imaging parameters: sagittal slice thickness, 1.0 mm with 50% overlap; no gap; repetition time of 9.9 ms; echo time of 4.6 ms; flip angle of 8; and matrix size of 240 × 240 px, resulting in 480 × 480 px image over a field view of 240 mm.
For individuals registered in the UKB, brain MRI scans were acquired at each of the three assigned sites using a 3.0-T Siemens Skyra MRI scanner.The scans included a T 1 -weighted sagittal 3D magnetization prepared rapid gradient echo image with the following imaging parameters: inversion time of 880 ms; repetition time of 2000 ms; voxel size of 1 × 1 × 1 mm 3 ; matrix size of 208 × 256 × 256; and SENSE factor (R) of 2.0 (Miller, 2016: https://www.nature.com/articles/nn.4393).
The inner and outer cortical boundaries were reconstructed using T 1 -weighted MRI scans by the Montreal Neurological Institute CIVET pipeline (http://www.bic.mni.mcgill.ca/ServicesSoftware/CIVET).The cortical morphological measures, such as sulcal depth, cortical thickness and grey/white intensity ratio, 32 were measured on the cortical surface at 81 924 vertices interconnected by 163 840 triangular edges.These metrics were subsequently re-sampled and mapped on to the surface template using the transformation derived from the surface registration to facilitate intersubject comparisons.

Graph convolutional neural network to predict the age of the entire brain and 11 different regions
The proposed brain age model was based on the graph-based convolutional networks (GCNs) introduced in our previous study 33 and was expanded for regional prediction (Fig. 1).The input to the GCNs was defined as a set of two feature vectors (cortical thickness and grey/white matter intensity ratio) at all vertices in a specified region of interest (ROI) and a sparse binary adjacency matrix representing the connections between each vertex and its neighbouring vertices.The output of the GCN model was the predicted brain age for each individual.The BAI was then calculated as the difference between the chronological age (age at scan) and the predicted brain age, which represents the relative brain health status of the individual.
To define the ROIs for which the BAIs were computed, the parcellation of the cortical surface into 65 cortical regions (ROIs) was performed using the AAL atlas.Yeo et al.'s atlas 34 was used to merge the initial ROIs into 10 functional network regions in the cerebral cortex.These regions consisted of the frontoparietal network, dorsal attention, ventral attention, sensorimotor, default mode, salient, language, auditory and visual and limbic network regions.This atlas was adapted and labelled on the Montreal Neurological Institute cortical surface template.Additionally, we included the Alzheimer's disease signature region to assess whether brain aging in this region was associated with APOE ɛ4.The Alzheimer's disease signature region has been defined in a previous study. 35To predict global brain age, we used the entire cortical surface as the ROI.

Statistical analyses
Student's t-test and χ 2 tests were used to compare the continuous and categorical variables.
To assess whether APOE ɛ2 or ɛ4 is associated with increased BAI values among the Korean and UK populations, linear regression analyses were performed with the APOE genotype (ɛ2 carriers, ɛ4 carriers and ɛ3 homozygotes) as a predictor after controlling for age, sex, hypertension and diabetes in the Korean and UK populations.To assess whether the association between the APOE genotype and BAI differed based on ethnicity, linear regression analyses were performed by adding a two-way interaction term (APOE genotype × ethnicity) to both populations.
To evaluate the combined effects of APOE ɛ4 and vascular risk factors (age, hypertension or diabetes) on BAI among the Korean and UK populations, linear regression analyses were performed with APOE ɛ4 carriers [ɛ3 homozygotes (reference)] as a predictor and APOE ɛ4 carriers × age, APOE ɛ4 carriers × hypertension or APOE ɛ4 carriers × diabetes as a two-way interaction term while controlling for age, sex, hypertension and diabetes in the Korean and UK populations.To determine whether the combined effects of APOE ɛ4 and vascular risk factors differed among ethnic groups, linear regression analyses were performed by adding a three-way interaction term (APOE ɛ4 carriers × age × ethnicity, APOE ɛ4 carriers × hypertension × ethnicity or APOE ɛ4 carriers × diabetes × ethnicity) in both populations.Additionally, to identify the regions where the combined effects of APOE ɛ4 and vascular risk factors might differ among ethnic groups, linear regression analyses were performed with regional BAIs as outcomes by adding a threeway interaction term in both populations.
For all statistical analyses, we applied the false discovery rate (FDR) adjustment to correct for multiple comparisons.
All reported P-values were two sided, with a significance threshold set at 0.05.All analyses were conducted using R version 4.3.0(Institute for Statistics and Mathematics, Vienna, Austria; www.R-project.org).

Demographics of CU populations in the UK and Korea
Table 1 presents the demographics of age-and sex-matched cohorts.The mean age did not differ between the Korean and UK populations (P = 0.441), and the female ratio did not Figure 1 GCN for regional brain age prediction.(A) The input to the GCN was defined as a set of two feature vectors (i.e.cortical thickness and grey/white matter intensity ratio) in all vertices in a specific ROI and a sparse binary adjacency matrix representing the connections between each vertex and its neighbouring vertices.The output of the GCN model was a predicted brain age for each individual.The BAI was calculated as the difference between chronological age (age at scan) and predicted brain age, which represented the relative brain health status of the individual.(B) We used ROIs as 10 functional network regions in the cerebral cortex.These regions consisted of the sensorimotor, FPN, dorsal and ventral attention, default mode, salient, language and auditory, visual network and limbic cortical regions.In addition, we included the Alzheimer's disease signature region to assess whether brain aging in this region was associated with APOE ɛ4.To predict global brain age, the entire cortical surface was used as the ROI.AD, Alzheimer's disease; APOE, apolipoprotein E; BAI, brain age index; FPN, frontoparietal network; GCN, graph-based convolutional network; ROI, region of interest.differ between the two groups (P = 0.550).However, the proportion of APOE ɛ4 carriers was lower in the Korean population (19.3%) than that in the UK population (25.7%;P < 0.001).Compared with the UK population, the Korean population had a higher prevalence of hypertension (41.9% versus 31.8%,P < 0.001) and diabetes (18.4% versus 6.7%, P < 0.001).

Effect of APOE genotype on BAI
BAI represents the difference between the chronological age from the predicted brain age (BAI = predicted brain age − chronological age).The APOE ɛ4 carriers in the Korean (β = 0.511, P = 0.012) and UK populations (β = 0.302, P = 0.006) showed increased BAI values compared with the APOE ɛ3 homozygotes (Fig. 2).There was no two-way interaction on BAI (P for ɛ4 × ethnicity = 0.405).In contrast, the APOE ɛ2 carriers in the Korean (β = 0.433, P = 0.122) and UK populations (β = 0.231, P = 0.198) did not show increased BAI values compared with the APOE ɛ3 homozygotes.

Discussion
In the present study, we conducted a systematic investigation to understand the effects of APOE ɛ4 allele on the BAI in relation to vascular risk factors, including age, hypertension and diabetes, in a large sample of CU Korean and UK populations.Our study yielded several key findings.First, both the Korean and UK populations demonstrated that APOE ɛ4 carriers showed an increased BAI compared with the APOE ɛ3 homozygotes, indicating a consistently greater deviation from the expected brain age across different ethnic groups.Second, the adverse effects of APOE ɛ4 on BAI increased with age in the Korean population, suggesting a cumulative impact on brain aging.In the UK population, the effect of APOE ɛ4 on BAI remained constant regardless of age.Lastly, the adverse effects of APOE ɛ4 on BAI were more pronounced in individuals with hypertension than in those without hypertension in the Korean population.In contrast, no significant interaction was observed between APOE ɛ4 and hypertension in the UK population.Overall, the combined effects of APOE ɛ4 and vascular risk factors, particularly age and hypertension, had a stronger influence on BAI in the Korean population than in the UK population.These findings underscore the need for ethnicity-specific strategies to manage vascular risk factors and mitigate APOE ɛ4-related brain aging in CU individuals.
Our first main finding indicated that APOE ɛ4 carriers showed an increased BAI compared with the APOE ɛ3 homozygotes in both Korean and UK populations.Only a few studies have investigated the relationship between APOE ɛ4 and BAI, and they have mainly focused on European and American populations. 19,20Considering that BAI is estimated based on brain atrophy, our findings align with those of previous studies conducted in Asian  populations.Previous studies performed in Asian populations also demonstrated that APOE ɛ4 contributes to increased brain atrophy in the medial temporal, posterior cingulate and insular regions. 27,36These findings further support our observation of the effects of APOE ɛ4 on BAI in the Korean and UK populations.maps represent the effect size of the interactions (grey: not significant after FDR correction).The generated maps indicate that age and hypertension had more detrimental effects on regional brain aging in individuals when combined with APOE ɛ4, particularly in the Korean cohort than in the UK cohort.With regard to age, these effects were significant in the language and Alzheimer's disease signature regions.The effects of hypertension were significant in the visual and limb regions.APOE, apolipoprotein E; KOR, Korea; UK, United Kingdom.
Our second major finding was that the impact of APOE ɛ4 on BAI was reinforced by aging in the Korean population, whereas in the UK population, the effects remained constant, regardless of age.This difference in the combined effects of age and APOE ɛ4 on BAI between ethnic groups may be attributed to socioeconomic disparities and historical factors, such as poverty and food shortages, during the exploited colonial era of the Japanese Empire (1910-45) and the Korean War (1950-53). 37Furthermore, before 1960, a smaller number of students in Korea received secondary education or higher education than those in the UK. 37,38These difficult experiences may have had lasting effects on their brain health and made them more vulnerable to the combined effects of aging and APOE ɛ4.A previous study suggested that childhood stress contributes to epigenetic changes in miRNA levels associated with Alzheimer's disease. 39Alternatively, the genetic differences between the Korean and UK populations could also play a role in the observed ethnic disparities.Previous studies from our group showed that variations in the gene encoding brain-derived neurotrophic factor are significantly associated with increased β-amyloid uptake in the brains of Korean individuals, but not in European individuals. 21Considering that polymorphisms in brain-derived neurotrophic factor may be associated with vulnerability in brain structure networks, 40 these genetic differences might contribute to accelerated brain aging resulting from the combined effects of aging and APOE ɛ4 in the Korean population compared with the UK population.Moreover, the deleterious effect of APOE ɛ2 on BAI increases with age in the Korean population.Although the exact pathomechanism underlying this relationship is not fully understood, the association between APOE ɛ2 and vasculopathy might explain the combined effects of aging and APOE ɛ2. 41urthermore, ɛ2 carriers had higher BAI values than those with ɛ3/ɛ3, but the difference was not significant.Our finding was consistent with that of a previous study that used measures of brain atrophy. 42Although APOE ɛ2 is a protective factor against the development of Alzheimer's disease through its amyloid burden-lowering effect, a previous systematic review revealed that APOE ɛ2 may be a risk factor for brain atrophy rather than a protective factor. 42inally, our study revealed an intriguing interaction between APOE ɛ4, hypertension and brain aging in the Korean population, which was not observed in the UK population.In the Korean population, the adverse effects of APOE ɛ4 on BAI were more pronounced in individuals with hypertension than in those without hypertension.These ethnic disparities in the combined effects of hypertension and APOE ɛ4 on BAI may be attributed to several factors.First, the effects of APOE ɛ4 and hypertension on brain health were more prominent in the Asian population than in the European population. 25,26,43For instance, hypertension is strongly associated with increased arterial stiffness 44 and carotid intima thickness 45,46 in Asians, which may further contribute to brain atrophy.In fact, interactions between hypertension-related arterial stiffness and APOE ɛ4 contribute to brain atrophy and cognitive decline. 47,48Additionally, studies have reported that the effects of APOE ɛ4 on the development of dementia are more prominent in Asians than in Europeans.Second, both APOE ɛ4 and hypertension are linked to the breakdown of the blood-brain barrier.Hypertension can lead to microvascular damage in the brain and disrupt the integrity of the blood-brain barrier.This disruption may impede the clearance of toxic substances associated with APOE ɛ4, such as amyloids, through impaired perivascular drainage, eventually resulting in increased brain atrophy and brain age. 49Therefore, the vulnerability of APOE ɛ4 and hypertension to brain health in Asian populations may underlie the observed ethnic differences in the combined effects of APOE ɛ4 and hypertension on the BAI.The intricate interplay between genetic factors, hypertension and brain aging requires further investigation to fully understand the underlying mechanisms and their implications for the provision of personalized healthcare and prevention of neurodegenerative diseases.
These interaction effects on the BAI were observed in specific functional brain network regions.Specifically, APOE ɛ4, age and ethnicity showed significant interaction effects on the regional BAI within the Alzheimer's disease signature and language network regions.This finding aligns with those of previous studies, which indicated that APOE ɛ4-related brain atrophy is predominantly observed in areas associated with Alzheimer's disease, such as the medial temporal and inferior parietal regions. 1,3These regions are vulnerable to the effects of APOE ɛ4.Additionally, APOE ɛ4, hypertension and ethnicity showed interaction effects on the regional BAI within the default and visual network regions.Previous studies have reported connectivity changes in the default mode network regions in individuals with preclinical Alzheimer's disease and hypertension, which can eventually contribute to cognitive decline. 50Therefore, the combined effects of APOE ɛ4 and hypertension may have specific effects on these functional brain networks.However, the specific implications of these interaction effects on the BAI within the specific brain networks have not yet been extensively investigated.Further studies are warranted to comprehensively understand the clinical significance and implications of these findings.

Limitations
The strengths of our study include the large sample size of the two cohorts and well-balanced clinical demographics between the two cohorts after propensity score matching.However, our study had several limitations.First, we did not consider Alzheimer's disease-specific biomarkers, including β-amyloid and tau, which are well-known mediators of APOE ɛ4 allele and BAI.Second, although we performed propensity score matching, the differences in the prevalence of vascular risk factors between the two ethnic cohorts may be potential confounders.Third, the number of APOE ɛ2 carriers was relatively small to identify the association between APOE ɛ2 allele and BAI.Fourth, although we have found the effect of sex on BAI in our previous study, 30 the present study was not conducted after stratifying by sex due to a relatively small number of several subgroups (e.g.female ɛ2 carriers with hypertension).Instead, we adjusted the sex as a confounder to mitigate the issue.Finally, our crosssectional study did not determine the causal effects of APOE ɛ4 allele and age/hypertension on BAI.Therefore, future studies are necessary to identify the neurodegenerative trajectory in relation to APOE ɛ4 and the presence of other risk factors.Nevertheless, our study is noteworthy as our results highlighted that the ethnic difference in APOE ɛ4 effect becomes significant with age and the presence of hypertension.

Conclusions
In conclusion, our study revealed ethnic differences in the combined effects of APOE ɛ4 genotype and vascular risk factors on brain age acceleration.These findings emphasize the need for ethnicity-specific strategies to mitigate accelerated brain aging in APOE ɛ4 carriers.Further research is needed to understand the underlying mechanisms and develop effective interventions to address accelerated brain aging in APOE ɛ4 carriers.

Figure 2
Figure 2 BAI distribution in each APOE genotype subgroup in the Korean and UK populations.The values depicted in the bar plot represent the mean BAI, and the values depicted in the error bar represent the standard error of the mean of each group.BAI = 0 indicates that the predicted brain age is equal to the chronological age, with higher values indicating an older-appearing brain than the chronological age.The P-values were obtained by linear regression analyses with APOE genotype (ɛ2 carriers, ɛ4 carriers and ɛ3 homozygotes) as a predictor after controlling for age, sex, hypertension and diabetes in the Korean and UK populations.The P-values for the interaction were obtained by linear regression analyses with the addition of a two-way interaction term (APOE genotype × ethnicity) to the covariates after controlling for age, sex, hypertension and diabetes in the Korean and UK populations.All P-values were modified after FDR correction for multiple comparisons.N represents the number of individuals used in the statistical analyses.APOE, apolipoprotein E; BAI, brain age index; FDR, false discovery rate; KOR, Korea; UK, United Kingdom.

Figure 3
Figure 3Differential interaction effects between APOE genotype and age on BAI among ethnic groups.BAI = 0 indicates that the chronological age at scan is the same as the predicted brain age, with higher values indicating an older-appearing brain than the chronological age.The P-values for two-way interaction were obtained by linear regression analyses with APOE ɛ4 or APOE ɛ2 carriers [ɛ3 homozygotes (reference)] as a predictor and the addition of each two-way interaction term (APOE ɛ4 carriers × age or APOE ɛ2 carriers × age) to the covariates after controlling for age, sex, hypertension and diabetes in the Korean and UK populations.The P-values for three-way interactions were obtained by linear regression analyses with the addition of each three-way interaction term (APOE ɛ4 carriers × age × ethnicity or APOE ɛ2 carriers × age × ethnicity) to the covariates after controlling for age, sex, hypertension and diabetes in the Korean and UK populations.All P-values were modified after FDR correction for multiple comparisons.N represents the number of individuals used in the statistical analyses.APOE, apolipoprotein E; BAI, brain age index; KOR, Korea; UK, United Kingdom.

Figure 4
Figure 4 Differential interaction effects between APOE genotype and hypertension on BAI among ethnic groups.The values depicted in the bar plot represent the mean BAI, and the values depicted in the error bar represent the standard error of the mean of each group.BAI = 0 indicates that the chronological age at scan is the same as the predicted brain age, with higher values indicating an older-appearing brain than the chronological age.The P-values for two-way interaction were obtained by linear regression analyses with APOE ɛ4 or APOE ɛ2 carriers [ɛ3 homozygotes (reference)] as a predictor and the addition of each two-way interaction term (APOE ɛ4 carriers × hypertension or APOE ɛ2 carriers × hypertension) to the covariates after controlling for age, sex, hypertension and diabetes in the Korean and UK populations.The P-values for three-way interaction were obtained by linear regression analyses with the addition of each three-way interaction term (APOE ɛ4 carriers × hypertension × ethnicity or APOE ɛ2 carriers × hypertension × ethnicity) to the covariates after controlling for age, sex, hypertension and diabetes in the Korean and UK populations.N represents the number of individuals used in the statistical analyses.APOE, apolipoprotein E; BAI, brain age index; HTN, hypertension; KOR, Korea; UK, United Kingdom.

Figure 5
Figure 5 Effects of the three-way interaction among APOE ɛ4, age or hypertension and ethnicity on regional brain age.T-value

Table 2 Interaction effects of APOE genotype and vascular risk factors on BAI Ethnicity Interactions between APOE genotype and vascular risk factors β (SE) P-value
APOE ɛ4 carriers × diabetes, APOE ɛ2 carriers × age, APOE ɛ2 carriers × hypertension or APOE ɛ2 carriers × diabetes) to the covariates after controlling for age, sex, hypertension and diabetes in the Korean and UK populations.Three-way interactions were analysed using linear regression models with the addition of each three-way interaction term (APOE ɛ4 carriers × age × ethnicity, APOE ɛ4 carriers × hypertension × ethnicity, APOE ɛ2 carriers × age × ethnicity or APOE ɛ2 carriers × hypertension × ethnicity) to the covariates after controlling for age, sex, hypertension and diabetes in the Korean and UK populations.APOE, apolipoprotein E; BAI, brain age index; UK, United Kingdom.